The process of this invention pertains to the conditioning of ferrous particles, and sintering and forming them into steel strip.
Although many ferrous-particle powders may have their oxide skins reduced by the techniques of this invention, the preferred particle is an as-water-atomized-ferrous-metal particle.
For clarity in describing the invention, the term "as water-atomized-ferrous-metal particles" refers to metal particles made by water atomizing and in the condition it comes out of a water atomizing apparatus without further processing or conditioning except for normal drying. That is, it has the irregular shape of as water-atomized-ferrous-metal particles with a martensitic structure, and it has whatever oxides and other impurities that occur in the as-water-atomized-ferrous-metal particles as it comes out of the water atomizing apparatus.
Agglomeration is defined herein as "any inter-particle bonding, visible to the naked eye, which persists through cooling and/or subsequent normal powder handling".
Typical techniques for making as-water atomized ferrous powder are described in United States patents of R. A. Huseby.
U.S. Pat. No. 3,325,227 for a "Method of Making Metal Powder" shows the particles have a high density and an irregular shape. Molten steel is fed by gravity in a downwardly moving stream. A plurality of flat sheets or curtains of water are impinged against the stream of molten steel at an angle greater than 5.degree. to atomize the stream of steel into ferrous particles with iron oxide surfaces.
U.S. Pat. No. 3,309,733 for "Apparatus for Producing Metal Powder" is directed to apparatus, including an improved nozzle assembly, for practicing the process of U.S. Pat. No. 3,325,277.
U.S. Pat. No. 3,334,408 to M. D. Ayers for "Production of Powder, Strip and Other Metal from Refined Molten Metal" teaches delivery of powder to compacting rollers. The compacted strip is then sintered and delivered to another set of compacting rollers.
The shape of powder particles is important in the forming of the particles into a handleable or conveyable sheet or strip. The strip compacted from the particles must have good strength and flexibility, whereby the particles from which it is formed should be mostly particles which have an irregular shape.
Water-atomized ferrous particles, such as those made by the Huseby process, are usually irregular in shape, but if the process is not properly controlled there are a substantial quantity of rounded or spherical particles. The as-water-atomized-ferrous-metal particles have an iron core of martensite which is particularly hard, with a skin of iron oxide having a small amount of iron-alloy oxides. Martensite is a hard, non-equilibrium structure which is formed by rapid cooling of particles produced from liquid steel. For particles of similar chemical composition, those having a martensite structure with a higher hardness than other particles are preferred for the process of this invention.
There are other techniques for fabricating ferrous metal powders. The powder may be produced by an electrolytic process which tends to produce a non-martensite dendritic or tree-shaped particle.
Some powders are produced by grinding the ferrous material. Grinding produces acicular (long and narrow), angular, or fragmented shaped particles of powder.
To provide ferrous metal powder which is suitable for producing sheets or strips of steel, it was formerly thought that the powder needs to be annealed before rolling so that the powder is soft enough that it can easily be formed.
Oxygen reductions can be effected during the annealing or softening stage of powder manufacturing operations, but such reductions are effected at relatively high temperatures, above 1200.degree. F. and usually between 1600.degree. and 1800.degree. F. The high temperatures are disadvantageous because at such temperatures the powder particles stick together or agglomerate. The agglomerated particles must then be cooled and tumbled, ground or comminuted, then screened before further processing.
Attention is again requested to the Ayers' U.S. Pat. No. 3,334,408, column 7, lines 53-55, wherein a reducing atmosphere is added to the powder in a holding bin. According to the patent, the reducing gas, such as hydrogen, is held by the particles for subsequent release during preheat of the powder, a process substantially different than claimed herein. See column 9, lines 17-19.